1. Field of the Invention
This invention relates to a dynamic pressure-type fluid-bearing apparatus and a polygon mirror-scanner unit with the fluid-bearing apparatus therein.
2. Description of the Prior Art
As the base oil of lubricating oils used in a conventional dynamic pressure-type fluid-bearing apparatus, there have been used synthetic hydrocarbons such as polybutene, poly-.alpha.-olephine, or the like. The reasons for their use are that these oils are not easily oxidized and do not easily rust metals; moreover, they absorb the scattering of the properties of bearings at the time of the mass-production of the bearing.
FIG. 1 shows a radial bearing that is an example of the dynamic pressure-type fluid-bearing apparatus in general. The radial bearing has a shaft 1 and a sleeve 2. A lubricant 3 is kept in the space between the shaft 1 and the sleeve 2 by capillary attraction. The tapers 2a of the sleeve 2, which define the bearing region with an effective width W, are provided for the purpose of keeping the lubricant 3 in the bearing region. Reference numeral la is a dynamic pressure-generating groove.
This dynamic pressure-type fluid-bearing apparatus operates as follows. A driving means (not shown) rotates either the shaft 1 or the sleeve 2. When the rotation begins, the pumping power of a dynamic pressure-generating groove 1a causes the centering of the shaft 1 with respect to the sleeve 2, so that the shaft 1 and the sleeve 2 are not in contact. The lubricant 3 undergoes shearing stress in the space between the shaft 1 and the sleeve 2, giving rise to heat. Because the shearing stress is the highest in the region of the bearing with the effective width W that is defined by the tapers 2a of the sleeve 2, the said region of the bearing with the effective width W becomes hotter than other regions. The heat produced therein is transferred via the shaft 1 and the sleeve 2 to the surroundings. When the amount of heat arising from the shearing stress and the amount of heat radiating from the entire dynamic pressure-type fluid-bearing apparatus are maintained at a certain level, there is a gradient of heat in the shaft 1 and the sleeve 2. In the same way, there is a temperature difference in the lubricant 3 between the region of the bearing with the effective width W and the region in the vicinity of the boundary of air and liquid. For that reason, a temperature gradient arises in the lubricant 3, as well. The occurrence of a temperature gradient causes a gradient in the surface tension, resulting in tangential stresses that correspond to the difference in surface tension, so that a flow begins in the lubricant 3. The direction of this flow is, as shown by the arrows in FIG. 2, from the direction of lower surface tension to the direction of higher surface tension. That is, the flow moves from the central area of the effective width W of the bearing to the outside directions.
The above-mentioned base oil of lubricating oils has a relatively large range of molecular weights. Moreover, the surface tension of the components of high molecular weight is larger than the mean surface tension of the lubricant overall, and the surface tension of the components of lower molecular weight is smaller than the mean surface tension of the lubricant overall, so that the components of high molecular weight segregate in the direction of low temperature and the components of low molecular weight segregate in the direction of high temperature.
Because of this phenomenon, the viscosity of the lubricant 3 in the region of the effective width W of the bearing decreases, causing a decrease in the stiffness of the bearing, which brings about swinging and eccentricity. This enhances the generation of heat still more, and both the temperature gradient and the gradient in the surface tension increase. The vapor pressure of the components of low molecular weight that have segregated to the central area of the bearing is lower than that of the lubricant overall, so cavitation occurs. Moreover, because of the increase in the gradient of surface tension, the lubricant is washed away toward the outside of the bearing, causing a deterioration of the properties of the bearing. In the worst situation, seizing occurs.